The present invention relates to signal reproducing methods using optical disk devices, optical disk devices which well eliminate non-linear distortions which can be generated in the recording/reproducing system thereof, signal processors of the optical disk devices and optical disk mediums.
FIG. 6 shows the basic structure of a recording/reproducing system of an optical disk device. User data 26 from a host device such as a controller 19 is converted through an encode/decode circuit 17 to a data row 27 of bits "1" and "0". A modulation system, for example, a 1-7 modulation system, converts user data 26 of two bits to a data row 27 of 3 bits, so that the interval of time T (sec) of a data row to be recorded is 2/3 (which is equal to the modulation rate) of the interval of time involving the user data 26. The modulated data row 27 includes 1-7 successive bits "0" and is then converted by a write current driver 16 to a data row 28 which does not change at a bit "0", but changes at a bit "1" alone. This data row 28 drives a laser source 38 such that a laser optical pulse is irradiated on an optical disk 22 in the recording process. When the laser power of the data row 28 is large, the laser pulse is recorded as a mark 11 on the disk 22 (that is, mark edge recording). An optical head 21 which has the laser source 38 and an optical system to irradiate a laser beam onto the optical disk is movable over the optical disk 22.
In reproduction, a photodetector 39 detects a light returning from the disk 22 to produce a detection signal 29, on the basis of which a read signal processor 68 forms a reproduced data row 7. The encode/decode circuit 17 decodes the data row 7 to the user data 26. In FIG. 7, reference numeral 72 denotes a bit cycle and 71, a determining level used to form the reproduced data row 7 by detection of an edge of the mark.
FIG. 7 shows the concept of the mark on the disk 22. The leading edge of the recorded mark 11 shifts especially under the influence of heat produced by the directly preceding recorded mark. This is called edge shift.
FIG. 8 shows the relationship between an edge shift quantity Le (m) and the length of the number of bits, B0, (bits) between adjacent marks. When B0 is small, a very large edge shift quantity Le (m) appears. Such a non-linear distortion in the recording process would be generated likewise when an opto-magnetic disk is used as a recording medium.
Conventionally, in order to prevent the generation of non-linear distortions in the recording process, a method of correcting and recording the edge position by assuming beforehand that a mark will shift, and a method of reducing the influence of heat on the next mark by rendering the laser power intermittent at the rear portion of the mark have been used. However, in those methods, the current fed to the laser is required to be controlled at very high speed with high accuracy. This renders the write current driver 16 complicated and would not provide sufficient performance due to fluctuations of the temperature of the device/disks, disadvantageously (non-linear distortions in the recording process).
FIG. 9 illustrates the principle of detecting recorded marks 11, 11' by applying optical spots 10, 10', 10" to the recorded marks 11, 11'. Reference numeral 73 denotes a reflected light detection signal corresponding to the mark 11; 73', that corresponding to the mark 11'; 75, that corresponding to the sum of marks (11+11'). Although reference numeral 74 denotes linear summation of signals 73, 73', but does not coincide with the actual signal 75. As just described above, since the actual reproduced waveform 75 does not coincide with the waveform 74 obtained by linear superposition of marks in the optical spots in the reproduction methods of detecting the edges of the marks 11, 11' from the signals 73 and 73', an accurate signal cannot be reproduced. This is because the relationship between the mark area and the intensity of the reflected light is not linear. This non-linearity cannot be corrected only by the linear equalizer included in the read signal processor 68 of FIG. 6--This is a problem of non-linear distortion involved in the reproduction of data in the optical disk.
This distortion also occurs likewise in an optical disk device dedicated to reproduction. Therefore, for example, even when a mark has been recorded without generation of a non-linear distortion, the generation of a non-linear distortion cannot be avoided in the reproduction when a method of detecting a mark with the intensity of the reflected light of the light spots is used.
It is known that non-linear distortions which can be produced in the recording process by the magnetic disk device are determined by the magnetic characteristic of a magnetic disk, the spacing between the magnetic disk and the recording head and the intensity of the recording magnetic field, and that the non-linear distortion is influenced by data preceding a bit to be recorded by one or two bits, which is disclosed on Institute of Electronics, Information and Communication Engineers of Japan, 1992, autumn meeting lecture papers, Vol. 5, page 35. According to this paper, a magnetic disk device, non-linear distortions in the recording process can be eliminated relatively easily by adjusting the position of a bit to be recorded on the basis of data preceding the bit to be recorded by one or two bits. Basically, any non-linear distortions are not produced in the reproducing process.
In such magnetic disk devices, application of a decision feedback equalizer using a random access memory (hereinafter referred to as a RAM-DFE) has been studied, the details of which are disclosed on IEEE Transaction of Communication, Vol. 39, No. 11 (1991), pp. 1559-1568. The object of use of a RAM-DFE in the magnetic disk device is to improve the S/N (signal/noise) ratio at the point of discriminating a reproduced signal by a combination of the RAM-DFE and a linear equalizer such as a traversal equalizer, and not to reduce the non-linear distortions basically.
It is an object of the present invention to provide an optical disk device which equalizes non-linear distortions, which may be generated in an optical disk device in recording and reproducing processes, such as those mentioned above, with high accuracy and at high efficiency, and an optical disk medium suitable for use with the optical disk device.
In order to achieve the above object in the present invention, an optical disk device which uses a light spot for recording and reproducing data includes non-linear equalizing means provided between photodetection means and decoding means to correct possible non-linear distortions generated in recording and reproducing the data to and from the optical disk.
In an optical disk device dedicated to reproduction, using a light spot for reproduction of data, non-linear equalizing means is provided between photodetection means and decoding means to correct possible non-linear distortions generated in the reproduction of data from the optical disk.
The non-linear equalizing means may include a decision feedback equalizer.
The non-linear equalizing means may include linearly equalizing means.
The optical disk device having the non-linear equalizing means may include learning control means for learning the equalizing characteristic of the non-linear equalizing means.
The linearly equalizing means may include a transversal equalizer which operates at a symbol rate. The number of taps Nf of the transversal equalizer may be: ##EQU1##
The register length Nb of the decision feedback equalizer may be: ##EQU2##
At this time, the reproduced spot diameter may be defined at a position where the light strength distribution on the optical disk surface becomes 1/e of the central light intensity of the optical spot.
The register length Nb of the decision feedback equalizer may be: ##EQU3##
In more detail, the number of taps Nf of the transversal equalizer in the optical disk may be: ##EQU4## where N (rps) is the disk rotational speed, D is the disk size (diameter) (inches), Bps (bytes/sec) is the data transfer rate; L.lambda. (m) is the laser wavelength used for reproduction; Na is the aperture number of an objective of the optical system; d is the data area ratio of a disk (d=the innermost peripheral track position/outermost peripheral track position); Bm (bits) is the minimum number of "0's" between adjacent marks in the modulation system; and Rate is the modulation rate.
The length Nb of the decision feedback equalizer may be: EQU Nb.gtoreq.(0.82 L.lambda./Na)/(.pi..times.D.times.d.times.0.0254.times.N.times.Rate/Bps/8) (Nf is a positive integer)
where N (rps) is the disk rotational speed, D is the disk size (diameter) (inches), Bps (bytes/sec) is the data transfer rate; L.lambda. (m) is the laser wavelength used for reproduction; Na is the aperture number of an objective of the optical system; d is the data area ratio of a disk (d=the innermost peripheral track position/outermost peripheral track position); and Rate is the modulation rate in the modulation system.
The register length Nb of the decision feedback equalizer may be: ##EQU5##
In the optical disk device, the data transfer rate may be constant through the radius of the optical disk.
The number of tracks through the radius of the optical disk may be divided by an positive integer such that the line recording density may be substantially constant at the respective track positions.
The linearly equalizing means may include means for correcting the linearly equalizing means in an adaptive manner in the reproduction.
The optical disk device may include a mechanism for receiving an optical disk removably.
An optical disk set in the optical disk device may have through the radius of the disk a plurality of learning training tracks to determine the characteristic of such non-linear equalizing means.
A signal processor and a signal processing integrated circuit of the optical disk device including the non-linear equalizing means may include learning control means for learning the equalizing characteristic of the non-linear equalizing means, memory means for storing a target signal used in the learning, and control input terminals for controlling the learning control means and memory means.
In the present invention, in the optical disk device using a light spot for recording/reproducing data, non-liner equalizing means is provided between the optical detecting means and the decoding means. Thus, the circuit, which corrects a recorded mark in the recording process, indispensable for the conventional structure is greatly simplified, so that possible non-linear distortions generated in the recording and reproducing processes in the optical disk are corrected appropriately in the reproducing system.
In the optical disk device dedicated to reproduction, using a light spot for reproduction of data, non-linear equalizing means is provided between the photodetection means and the decoding means. Thus, even when the diameter of the light spot is several times that of the recorded mark, non-linear distortions which can be generated in that case are corrected appropriately.
Provision of the decision feedback equalizer of FIG. 1 using a RAM in the non-linear equalizing means serves to efficiently correct possible non-linear distortions generated in the recording and reproducing processes with a relatively simple circuit structure.
Provision of linearly equalizing means in the non-linear equalizing means serves to improve the quality of a reproduced signal and the accuracy of discrimination.
Provision of learning control means to learn the equalizing characteristic of the non-linear equalizing means in the optical disk device including the non-linear equalizing means appropriately maintains the equalizing characteristic even when the device temperature and characteristic and hence the non-linear distortions change.
When the linearly equalizing means is a transversal equalizer which operates at a symbol rate, and the linearly equalizing means is realized by the transversal equalizer on the basis of the minimum number of successive bits "0" determined by the modulation system, the shortest bit length determined by the modulation system, and the reproduced spot diameter of the laser beam, the minimum number of taps Nf of the transversal equalizer is determined as follows:
FIG. 2 shows the relationship between the shortest mark length Lpmin and the diameter of the reproduced spot of the laser beam.
Let Rs (m) be the diameter of the reproduced spot of the laser beam; let N (rps) be the rotational speed of the optical disk; let R (m) be a radial position on the disk, data from which is to be reproduced; let 1/T (bits/sec) be the data rate at that position after the modulation; and let Bm (bits) be the minimum number of successive bits "0" of the modulation system. Since the line velocity V (m/sec) at the position of the reproduction is: EQU V=2 .pi.RN (m/sec),
the shortest mark length Lpmin (m) determined from the relationship with the modulation system is given by EQU Lpmin-(Bm+1).times.VT (m).
In FIG. 2, the reproduced signal is present from the time when the reproduced spot 10 starts to sweep a mark 11 to the time when the reproduced spot 10 leaves the mark 11 or to the time when the reproduced spot 10 arrives at the spot 10". The movement range Lspmin (m) of the reproduced spot where the reproduced signal is present is: EQU Lspmin=Lpmin+Rs (m).
The transversal equalizer is required to equalize an area Lf which is 1/2 of the range Lspmin where the reproduced signal is present, as shown in FIG. 2. The signal in the area Lb after the area Lf, inclusive of waveform distortions generated by the transversal equalizer, is equalized by the feedback equalizer. Thus, the number of taps Nf of the transversal equalizer is: ##EQU6##
Rewriting this result, using the minimum number of successive bits "0" determined by the modulation system, the shortest bit length determined by the modulation system, and the reproduced spot diameter of the laser beam: ##EQU7##
By this setting, the tap coefficient is set appropriately and an improvement to the S/N ratio at the discriminating point is expected by the effect of the non-linear equalizing means. The number of taps cannot be defined in the reproducing system of the magnetic disk device.
As shown in FIG. 9, the register length Nb of the decision feedback equalizer which removes possible non-linear distortions in the reproduction generated because the relationship between the mark area and the reflected light intensity is non-linear as shown in FIG. 9 is determined as follows:
Now, the trailing edge of the minimum mark is equalized in consideration of the length of the mark which will influence the reproduced spot 10, as shown in FIG. 2. To this end, the required register length Nb of the decision feedback equalizer is given by: EQU Nb.gtoreq.Rs/(2 .pi.RNT) (Nb is a positive integer)
where Rs (m) is the reproduced spot diameter, and 2 .pi.RNT (m) is the bit length.
That is, if the following conditions are satisfied: EQU Nb.gtoreq.the reproduced spot diameter/the shortest bit length (Nb is a positive integer),
the non-linear distortions in the reproduction are corrected efficiently.
At this time, if the reproduced spot diameter is defined at a position where the light intensity distribution on the optical disk surface is 1/e of the central light intensity of the light spot, device design is achieved which does not depend on various parameters of the optical system.
In order to eliminate a non-linear edge shift quantity Le (m) determined by a recorded mark pattern as shown in FIG. 8, the maximum length Bm (bits) of the number of bits between adjacent marks determined by the modulation system is required to be considered. The register length Nb (taps) of the decision feedback equalizer to correct the non-linear edge shift quantity is: EQU Nb.gtoreq.Bm+1 (taps) (Nb is a positive integer).
That is, if ##EQU8## possible non-linear distortions generated in the recording process are corrected efficiently. Since the edge shift quantity Le (m) rapidly decreases as the number of bits B0 between adjacent marks increases, it is obvious that the conditions of the Nb are not necessarily required.
In more detail, if the number of taps Nf of the transversal equalizer in the optical disk is: ##EQU9## where N is the disk rotational speed (rps), D is the disk size (diameter) (inches), Bps (bytes/sec) is the data transfer rate; L.lambda. (m) is the laser wavelength to be used for reproduction; Na is the aperture number of an objective of the optical system; d is the data area ratio of a disk (d=the innermost peripheral track position/outermost peripheral track position); and Bm (bits) is the minimum number of successive bits "0's" in the modulation system and Rate is the modulation rate, the S/N ratio is improved by setting the tap coefficient at an appropriate value and using the effect of the decision feedback equalizer.
If the length Nb of the decision feedback equalizer is given by the expression below, an optical disk device dedicated to reproduction is constructed in which non-linear distortions in the reproduction process are corrected efficiently: EQU Nb.gtoreq.(0.82L.lambda./Na)/(.pi..times.D.times.d.times.0.0254.times.N.tim es.Rate/Bps/8) (Nf is a positive integer)
where N is the disk rotational speed (rps), D is the disk size (diameter) (inches), Bps (bytes/sec) is the data transfer rate; L.lambda. (m) is the laser wavelength to be used for reproduction; Na is the aperture number of an objective of the optical system; d is the data area ratio of a disk (d=the innermost peripheral track position/the outermost peripheral track position); and Rate is the modulation rate in the modulation system.
If the register length Nb of the decision feedback equalizer is: ##EQU10## an optical disk device is constructed in which possible non-distortions generated in the writing process are corrected efficiently and which data is writable additionally and rewritable.
If in the optical disk device the data transfer rate is maintained constant through the radius of the optical disk, device control is further facilitated.
If the number of tracks through the radius of the disk is divided by an positive integer and the line recording density is substantially constant at the respective track positions, the equalizing characteristic of the non-linear equalizing means is shared by many tracks and the capacity of a memory which stores coefficient values determining the equalizing characteristic is reduced.
If the linearly equalizing means includes means for correcting non-linear distortions in an adaptive manner in the reproduction, improved appropriate equalizing characteristic is provided and the S/N ratio at the discriminating point is improved.
If the optical disk device includes a mechanism for receiving an optical disk removably, an optical disk device compatible considerably easily with different optical disks is provided even when fluctuations in the characteristic of the optical disk.
If an optical disk used in the optical disk device has a learning training track to determine the characteristic of a plurality of such non-linear equalizing means through the radius of the optical disk, an appropriate equalizing characteristic is learned each time the disk is set in the disk device. As a result, the storage capacity of the disk is increased.
If a signal processor and a signal processing integrated circuit of the optical disk device having the non-linear equalizing means include learning control means for learning the equalizing characteristic of the non-linear equalizing means, memory means for storing a target signal used in the learning, and control input terminals for controlling the learning control means and the memory means, a target signal always correct is given to the learning control means in the learning process even in the initial equalizing characteristic where errors in the discrimination will occur very frequently. Thus, higher accuracy equalizing characteristic is realized with high convergency and as a result a high reliability optical disk device is constructed.